1. Field of the Invention
The present invention relates to a liquid crystal display (or "LCD") device which provides a structure that allows for detection of the distortion of the common voltage resulting from a delay time in application of the common voltage. More specifically, the present invention relates to a liquid crystal display device including a common line which eliminates the need for a common drive circuit and which is arranged to allow for detection of an amount of distortion of the common voltage so that such distortion can be corrected.
2. Description of the Related Art
As seen in FIG. 1, conventional LCDs include two transparent panels, i.e., an upper panel 11 and a lower panel 10, and a liquid crystal material 15 injected between the upper panel 11 and the lower panel 10. The lower panel 10 includes a plurality of scan lines 24 and a plurality of data lines 23. The scan lines 24 and data lines 23 are arranged in a matrix form wherein the scan lines 24 and the data lines 23 intersect each other. At each intersecting point between the scan lines 24 and the data lines 23, a thin film transistor 21 (or "TFT") and a pixel electrode 20 are provided. The TFT 21 comprises a gate electrode 25, a source electrode 27 and a drain electrode 29. Each gate electrode 25 is connected to one of the scan lines 24 and each source electrode 27 is connected to one of the data lines 23. Each drain electrode 29 is connected to a respective pixel electrode 20. Although not shown in FIG. 1, one end of each scan line 24 is connected to an output of a scan driving IC and one end of each data line 23 is connected to an output of a data driving IC. Additionally, the lower panel 10 can comprise a portion of a common line carrying a common signal applied from an outer device.
As shown in FIG. 1, the upper panel 11 comprises a common electrode 14 and a color filter 13. There are three kinds of the color filters 13, a red filter (R), a green filter (G) and a blue filter (B). Each of the color filters 13 is disposed at a position corresponding to the position of the pixel electrode 20 in the lower panel 10. The common electrode 14 is formed on the color filter 13. Generally, the common electrode 14 is made with one body covering one of the major surfaces of the upper panel 11. Alternatively, the common electrode 14 comprises many bodies which are formed as strips arranged along with the scan lines 24 or the data lines 23.
A molecule array direction of the liquid crystal material 15 injected between the upper panel 10 and the lower panel 11 is changed by the voltage difference between the pixel electrodes 20 and the common electrode 14. When a scan voltage is applied to the scan lines 24, the TFT 21 is turned on by the voltage applied to the gate electrode 25. At that time, a data voltage applied to the data line 23 is sent to the drain electrode 29 from the source electrode 27 of the TFT 21 so that the data voltage is applied to the pixel electrode 20 which is connected to the drain electrode 29. Consequently, a voltage difference is generated between the pixel electrode 20 and the common electrode 14. As a result of the change of the molecule array direction of the liquid crystal material, the LCD functions as a display device.
FIGS. 2a and 2b show the conventional structure of the common electrode 14 in a liquid crystal display device. Referring to the FIGS. 2a and 2b, the common pads 59a and 59b of the scan driver IC 55 and the data driver IC 56 apply a common voltage for the lower panel 10 to the common electrode 14 formed in the upper panel 11. A silver dot 40 (or Ag dot) is disposed in each of the four corners of the lower panel 10 in contact with the common electrode 14 and to the scan driver IC common pad 59a and the data driver IC common pad 59b so that the common voltage is applied to the common electrode 14 through the Ag dots 40.
In the LCD according to the above-mentioned structure, the wave form of the common voltage which is detected at the portion of the common electrode 14 located near the Ag dot is different from the wave form of the common voltage which is detected at the portion of the common electrode 14 located spaced from the Ag dot. The difference in wave form is caused by a signal delay which occurs because the common voltage signal is delayed by the resistivity of the common electrode 14. Therefore, it has been necessary to provide a third drive IC 57 for controlling the common electrode 14.
In order to solve this problem without using a third drive circuit, U.S. Pat. No. 5,311,342 describes a structure, as shown in FIG. 3, in which an expanded common line 30 is formed continuously along two adjacent side portions of the matrix display portion of the LCD. The expanded common line 30 surrounds the matrix display portion by extending continuously from the scan pad part located at the lower left corner of the matrix portion in FIG. 3 to the data pad part located at the upper right corner of the matrix portion in FIG. 3 so that the scan pad part and the data pad part are connected each other. The common line 30 disposed in the lower panel 10 and the common electrode 14, shown as a dashed-line square in FIG. 3 and disposed in the upper panel 11, are connected to each other through the Ag dot 40. Additionally, a conductive line 50 which is separately formed with the common line is connected to the common electrode 14 in order to reduce the time delay of the common voltage.
However, the structure described in U.S. Pat. No. 5,311,342 experiences problems caused by the expanded common line 30 which is continuously formed to extend from the scan driver IC common pad 59a located at the lower left corner of the matrix portion in FIG. 3 to the data driver IC common pad 59b located at the upper right corner of the matrix portion of FIG. 3. Because of the common line 30 extending continuously between the data pad part and scan pad part, it is not possible to detect the exact amount of the delay time of the common voltage at the center of the display area. So, it is not possible to calculate the amount of the drop down voltage of the common voltage resulting from the time delay. Because such calculation of the drop down voltage is not possible, there is no way to compensate and correct for the drop down voltage. These problems result in flicker being generated in the image displayed by the device shown in FIG. 3 and also prevent uniformity of contrast and brightness in images displayed in the device of FIG. 3.